A constant-temperature anemometer using a hot wire is known from U.S. Pat. No. 3,747,577. Hot-wire air-mass sensors of this type operate very accurately and have been perfected for mass production. The principle of operation of hot-wire air-mass sensors is the cooling of the hot wire as a function of the air-flow rate. However, their use becomes problematical if the direction of flow is reversed because the cooling effect of the hot wire is independent of the direction of flow; this means that the hot-wire air-mass sensor measures the total air flow, whereas for use in a motor vehicle, only the air flowing in the direction of the internal combustion engine is of relevance.